//! An [Index to Location Table](https://docs.microsoft.com/en-us/typography/opentype/spec/loca)
//! implementation.

use core::convert::TryFrom;
use core::num::NonZeroU16;
use core::ops::Range;

use super::head::IndexToLocationFormat;
use super::*;

/// An [Index to Location Table](https://docs.microsoft.com/en-us/typography/opentype/spec/loca).
#[derive(Clone, Copy, Debug)]
pub enum Table<'a> {
	/// Short offsets.
	Short(LazyArray16<'a, u16>),
	/// Long offsets.
	Long(LazyArray16<'a, u32>),
}
impl<'a> Table<'a> {
	/// Parses a table from raw data.
	///
	/// - `num_glyphs` is from the `maxp` table.
	/// - `format` is from the `head` table.
	pub fn parse(num_glyphs: NonZeroU16, format: IndexToLocationFormat, data: &'a [u8]) -> Option<Self> {
		// The number of ranges is `maxp.numGlyphs + 1`.
		//
		// Check for overflow first.
		let mut total = if num_glyphs.get() == u16::MAX {
			num_glyphs.get()
		} else {
			num_glyphs.get() + 1
		};

		// By the spec, the number of `loca` offsets is `maxp.numGlyphs + 1`.
		// But some malformed fonts can have less glyphs than that.
		// In which case we try to parse only the available offsets
		// and do not return an error, since the expected data length
		// would go beyond table's length.
		//
		// In case when `loca` has more data than needed we simply ignore the rest.
		let actual_total = match format {
			IndexToLocationFormat::Short => data.len() / 2,
			IndexToLocationFormat::Long => data.len() / 4,
		};
		let actual_total = u16::try_from(actual_total).ok()?;
		total = total.min(actual_total);

		let mut s = Stream::new(data);
		match format {
			IndexToLocationFormat::Short => Some(Table::Short(s.read_array16::<u16>(total)?)),
			IndexToLocationFormat::Long => Some(Table::Long(s.read_array16::<u32>(total)?)),
		}
	}
	/// Returns the number of offsets.
	#[inline]
	pub fn len(&self) -> u16 {
		match self {
			Table::Short(ref array) => array.len(),
			Table::Long(ref array) => array.len(),
		}
	}
	/// Checks if there are any offsets.
	pub fn is_empty(&self) -> bool {
		self.len() == 0
	}
	/// Returns glyph's range in the `glyf` table.
	#[inline]
	pub fn glyph_range(&self, glyph_id: GlyphId) -> Option<Range<usize>> {
		let glyph_id = glyph_id.0;
		if glyph_id == u16::MAX {
			return None;
		}
		// Glyph ID must be smaller than total number of values in a `loca` array.
		if glyph_id + 1 >= self.len() {
			return None;
		}
		let range = match self {
			Table::Short(ref array) => {
				// 'The actual local offset divided by 2 is stored.'
				usize::from(array.get(glyph_id)?) * 2..usize::from(array.get(glyph_id + 1)?) * 2
			}
			Table::Long(ref array) => {
				//
				usize::num_from(array.get(glyph_id)?)..usize::num_from(array.get(glyph_id + 1)?)
			}
		};
		if range.start >= range.end {
			// 'The offsets must be in ascending order.'
			// And range cannot be empty.
			None
		} else {
			Some(range)
		}
	}
}
